1. Field of the Invention
The present invention relates to integrated circuits (ICs), and, in particular, to resistors for integrated circuits.
2. Description of the Related Art
Many integrated circuits designed to process analog signals use differential structures such as operational amplifiers (op-amps) which generate output signals that are proportional to the difference between two input signals. One advantage of differential structures is that they have the potential ability to cancel out even harmonics in the input signals.
In some configurations, a single input signal is converted into two input signals for a differential circuit. For example, the single input signal may be used as one input signal to the differential circuit, while an inverted version of that input signal is generated for use as the other input signal to the differential circuit. One conventional way to generate this second input signal is to use a unity-gain inverting amplifier to generate the negative of the original input signal.
Referring now to FIG. 1, there is shown a circuit diagram of conventional inverting amplifier 100 comprising operational amplifier 102, feedback resistor R.sub.F 104, and source resistor R.sub.S 106. In some IC implementations, resistors 104 and 106 are diffused resistors. Because diffused resistors have non-zero voltage coefficients, the resistance values of resistors 104 and 106 will vary with the voltage level between the resistor and the underlying material. As a result, the gain of inverting amplifier 100 will tend to deviate from the desired gain over the operating voltage range of the circuit. This means that the inverted signal V.sub.O (i.e., the second input signal to the differential circuit described earlier) will not always have the same magnitude as the original signal V.sub.IN (i.e., the first input signal to the differential circuit). In general, both odd and even harmonics are introduced.
One possible solution to this problem is to use polysilicon resistors, which have relatively small voltage coefficients, instead of diffused resistors. Unfortunately, polysilicon resistors tend to have low sheet resistance and may therefore require relatively large areas to implement.
One aspect of the present invention is to provide resistors having relatively small voltage coefficients for use in integrated circuits. Another aspect is the advantage of generating smaller odd and even harmonics than conventional IC resistor circuits.
Further aspects and advantages of this invention will become apparent from the detailed description which follows.